1. Field of the Invention
The field of the invention concerns a frequency tripled laser, and in particular a method and apparatus for generating a frequency tripled beam.
2. Background of the Invention
Optical harmonic generation using a non-linear medium such as a crystal, provides a method for doubling, tripling or more generally producing the Nth harmonic of the frequency of electromagnetic radiation emitted by a laser or other high intensity source. The third harmonic is produced by sum frequency generation of the laser fundamental and its second harmonic beams through second order nonlinearities of the nonlinear medium (P (.omega.)=.eta..sub.0 (3)*P .sub.0 (.omega.)*P .sub.0 (2.omega.)). The common third harmonic generation method, which is known in the art, combines the fundamental and the second harmonic beam in a nonlinear crystal to generate its third harmonic in the external laser cavity. For a high power laser, typical efficiency is limited to less than 25% overall conversion efficiency, since unconverted fundamental beam and second harmonic beam are wasted in that configuration. The overall conversion efficiency .eta.=P(3.omega.)/P.sub.0 (.omega.)=.eta..sub.0 (3.omega.)*.eta..sub.1 (2.omega.)P.sub.0.sup.2 (.omega.)-.eta..sub.1 (2.omega.)P.sub.0.sup.2 (.omega.)) is less than 25% for even very high power pulsed Nd:YAG lasers (see Continuum Products brochures).
The theory of intra-cavity second harmonic generation (SHG) has been discussed by R. G. Smith, I.E.E.E. Journal of Quantrum Electr. QE-6, 215-223 (1970). Franken and Ward , in Rev. Mod. Phys. 35 23 (1963) has noted that a crystalline medium can produce a second harmonic of the frequency of the incident radiation only if the crystal lacks inversion symmetry so that the second order polarization tensor d.sup.ij in the lowest order non-linear polarization term does not vanish. If the crystal has inversion symmetry, the lowest order non-linear polarization contribution is cubic in the electric field strengths, and this crystal will produce only third or higher harmonics of the initial frequency. Beam or parametric production of harmonics has been demonstrated in crystals such as LiNbO.sub.3, BaNa(NbO.sub.3); LiO.sub.3, KDP, KTiOPO.sub.4, BBO and LiB.sub.3 O.sub.5. LiB.sub.3 O.sub.5 is also referred to as LBO crystal.